The proliferative potential of normal mammalian somatic cells is limited, with the vast majority daughter cells eventually entering either a senescent state or undergoing apoptosis. In rodent cell cultures, the emergence of immortalized cell lineages also occurs at a readily detectable frequency. To evaluate the roles of gene products that modulate higher order chromatin structure in these growth regulatory processes, histone acetyltransferases and deacetylases, as well as chromodomain-containing proteins linked to epigenetic inheritance, have been examined. Proteins of the latter class, specifically M31 and M32, were demonstrated to extend proliferative potential under conditions of oxidative stress. Such findings suggest a direct link between senescence and the integrity of higher-order chromatin-mediated silencing